Sym  Comment  Sym  Comment 
A_{ }  Heat transfer area  ${\dot{q}}_{incupper}$  Solar radiation on absorber’s upper half 
${A}_{c}$  Concentrator’s captation area  ${\dot{q}}_{net}$ _{ }  Net heat absorbed by the absorber 
c  Specific heat  ${\dot{q}}_{radabs}$  Absorber’s radiative heat loss 
D  Diameter of absorber tube  ${\dot{q}}_{radem}$ _{ }  Emitted radiation 
e  Enclosure’s wall thickness  ${\dot{q}}_{radrec}$ _{ }  Received radiation 
${F}_{ij}$ _{ }  View factor between surfaces “i” and “j”  ${\dot{q}}_{radwe}$  Radiative heat transfer to outer face of enclosure’s wall 
g_{ }  Gravity  ${\dot{q}}_{radwi}$  Radiative heat transfer to inner face of enclosure’s wall 
h_{ }  Convective heat transfer coefficient  ${\dot{q}}_{s}$ _{ }  Sensible heat of enclosure’s walls 
${h}_{e}$ _{ }  Outerface convective heat transfer coefficient of enclosure’s wall  ${\dot{q}}_{sol}$  Solar irradiance 
${h}_{i}$  Innerface convective heat transfer coefficient of enclosure’s wall  r  Radius of absorber tube 
k_{ }  Thermal conductivity  R_{ }  Concentrator’s reflectivity 
L_{ }  Characteristic longitude  Ra_{ }  Rayleigh number 
${L}_{opt}$ _{ }  Collector’s optical losses  ${T}_{a}$ _{ }  Environmental temperature 
m_{ }  Mass  ${T}_{abs}$  Absorber’s surface temperature 
Nu  Nusselt number  ${T}_{w}$ _{ }  Central temperature of enclosure’s wall 
Pr  Prandtl number  ${T}_{wex}$ _{ }  Outerface temperature of enclosure’s wall 
PTC  Parabolic trough collector  ${T}_{win}$ _{ }  Innerface temperature of enclosure’s wall 
${\dot{q}}_{convabs}$  Absorber’s convective heat loss  $\beta $  Volumetric expansion coefficient 
${\dot{q}}_{convwe}$  Convective heat transfer to outer face of enclosure’s wall  $\epsilon $ _{ }  Emissivity 
${\dot{q}}_{convwi}$  Convective heat transfer to inner face of enclosure’s wall  ${\epsilon}_{ij}$ _{ }  Effective emissivity between surfaces “i” and “j” 
${\dot{q}}_{dir}$ _{ }  Direct solar radiation  $\nu $ _{ }  Kinematic viscosity 
${\dot{q}}_{inc}$ _{ }  Incident solar irradiation  $\sigma $ _{ }  StefanBoltzmann constant 
${\dot{q}}_{inclower}$ _{ }  Solar radiation on absorber’s lower half  _{ } 
